The present invention relates to a steam generator which generates steam by heating water, particularly to that suitable for forming a silicon oxide layer by a steam oxidation process.
In the process of producing semiconductor integrated circuits, a silicon oxide layer is often formed by oxidizing the silicon substrate itself. Various processes for oxidizing the surface of a silicon substrate have been put into practice. Among them is the steam oxidation process wherein SiO2 is formed by exposing a heated silicon substrate to steam generated by heating pure water reserved in a container. The steam oxidation process is advantageous in its high oxidation speed, and therefore a thick oxide layer can be formed in a relatively short time.
In a conventional steam oxidation process, pure water in a container is heated by a heater placed at the bottom of the container. Therefore, when boiling, the steam bubbles generated through submerge boiling rise and burst at the water surface, which scatters water by small droplets. If the water bumps, such water droplet scattering is more vigorous. Such water droplets are involved in the stream of steam and delivered to the silicon surface. Bumping also causes fluctuation of flow rate of the steam. Such water droplet scattering and flow rate fluctuation are of course harmful to the formation of a silicon oxide layer especially when growing a thick and uniform oxide layer, in which case a stable delivery of steam is required for a long time.
In a general steam generator, some flow control mechanism such as a flow control valve is provided at the outlet of the container to stabilize the flow rate of the steam. The flow control mechanism is not appropriate in the steam oxidation process, however, because impurities may be eluted from the flow control mechanism and enter the water, which seriously deteriorates the quality of the produced silicon substrate. In addition to that, when a flow control valve is used, there must be a pressure difference across the valve, and the pressure in the container must be higher. This requires a high pressure resistance of the container. Further, such high pressure in the container increases the boiling point of the water and enhances elution of the material, or impurities, from the container.
Even if the fluctuation in the flow rate due to bumping is negligible, some measures should be taken to control the flow rate of the steam delivered. Instead of using the above-described flow control mechanism, it is possible to control the electric power to the heater for controlling the flow rate of the steam. This, however, is inadequate because the control response is slow due to the large heat resistance and heat capacity of the container (which is normally made of quartz glass). The response becomes slower as the amount of the water is increased in the container. That is, when water is supplied into the container to make up for the evaporated water, the temperature of the water falls slightly, which largely decreases the evaporating speed (or the steam generating speed). It will take some time to recover the proper steam generating speed.
The present invention is achieved in view of the above problems. One of the objects of the present invention is, therefore, to provide a steam generator which can generate steam of a stable flow rate without using a steam flow control mechanism. Another object is to provide a steam generator generating a high quality steam which is almost free from impurities or water droplets.
The first steam generator according to the present invention is characterized in that it comprises:
a) means for generating a microwave radiation;
b) a waveguide for the microwave radiation;
c) a structured body placed in the waveguide to absorb water; and
d) means for supplying water to the structured body.
The second steam generator according to the present invention is characterized in that, in the above first steam generator, the water is pure water.
The third steam generator according to the present invention is characterized in that, in the above first steam generator, it further includes:
e) means for detecting the temperature of the structured body; and
f) means for controlling the microwave generating means to regulate the energy of the microwave radiation sent out to the waveguide according to the temperature detected by the temperature detecting means.
The fourth steam generator according to the present invention is characterized in that, in the above first steam generator, the waveguide is closed at an end and the steam generator further includes:
g) means for detecting the temperature of the microwave generating means; and
h) means for controlling the microwave generating means to regulate the energy of the microwave irradiation sent out to the waveguide according to the temperature detected by the temperature detecting means.
The fifth steam generator according to the present invention is characterized in that, in the above first steam generator, the structured body is a solid body including a large number of micro-pores connected to each other.
The sixth steam generator according to the present invention is characterized in that, in the above first steam generator, the structured body is placed in a container having a water inlet and a steam outlet at the upper part and a water drain at the lower part. The water inlet, steam outlet and water drain are connected to respectively appropriate pipes outside of the waveguide.
The seventh steam generator according to the present invention is characterized in that, in the above sixth steam generator, the container is made of quartz.
In the first steam generator according to the present invention, the microwave generating means sends out microwave radiation into the waveguide, while a water supplying means supplies water to the structured body. The water quickly infiltrates into the structured body and is retained there. The water is heated by the microwave radiation coming to the structured body. The water temperature rises in a short time, and the water evaporates and the volume increases. The steam generated in the structured body passes through the micro-pores connected to each other and is emitted from every surface of the structured body. When steam contacts cool water in passing through the micro-pores, the steam is deprived of its latent heat and is condensed to water. Thus the entire absorbing body is heated uniformly, which enables a homogeneous steam generation from the whole structured body.
In the first steam generator, all the water supplied to the structured body can be converted to steam by appropriately determining the flow rate of the water supplied to the structured body and the energy of the microwave irradiation sent out to the waveguide. This means that the flow rate of steam delivered corresponds to the flow rate of water supplied, and it is possible to control the flow rate of steam by controlling the flow rate of the water. The flow rate of water can be controlled by, for example, a flow control valve. If the water supplying means is realized by a water pump, it is possible to control the water pump to control the flow rate of the water.
According to the first steam generator, it is not necessary to provide a flow controller at the outlet of the steam generator in order to control the flow rate of the steam. It is necessary, though, to provide some flow control mechanism, such as a flow control valve, at the water inlet of the steam generator. But the temperature of the water entering the steam generator is very low compared to that of the steam, so that impurities hardly elute from the flow control valve.
In the first steam generator, further, the water is not heated by the structured body, but is heated directly by the microwave radiation, so that very few impurities elute from the structured body. Thus, as in the second steam generator according to the present invention, by supplying pure water free from impurities to the structured body, very clean steam free from impurities can be generated. Further, by controlling the flow rate of the water supply, the flow rate of the delivered steam can be stabilized. Response time in controlling the flow rate of steam is very short because the water evaporates momentarily when supplied to the structured body. Besides, the generated steam is very homogeneous since no bumping occurs and there is no water droplet scattering.
In the first or second steam generator, the temperature rise of the structured body is small as long as water is supplied to it because the microwave radiation is used to heat the water, and the water deprives the structured body of the latent heat of vaporization. If, however, water is not supplied to the structured body, the structured body itself absorbs the microwave radiation and the temperature rises. Further it is a waste of energy to generate microwave radiation when there is no water in the structured body.
In the third steam generator according to the present invention, when the temperature detected by the temperature detecting means is above a preset value, the controller controls the microwave generating means to reduce energy intensity of the microwave radiation. This prevents overheating of the structured body and save the power consumption when water is not supplied. Various devices are available for the temperature detecting means. Among them, a non-contact type temperature sensor is preferable. For example, an infrared temperature sensor is useful, which detects an infrared radiation emitted from the heated structured body.
If an end of the waveguide is closed and no water is supplied to the structured body, the microwave radiation which is not absorbed by the structured body is reflected by the end and return to the microwave generating means. This may cause overheating of the microwave generating means and may lead to its failure.
In the fourth steam generator according to the present invention, similar to the above-described third steam generator, when the temperature detected by the temperature detecting means is above a preset value, the controller controls the microwave generating means to reduce energy intensity of the microwave radiation. This prevents overheating of the structured body or the microwave generating means, and suppress the power consumption when water is not supplied.
In the fifth steam generator according to the present invention, supplied water quickly infiltrates into the structured body owing to the capillary action. The structured body can be made of quartz glass or synthetic resin such as plastics including a large number of micro-pores connected to each other, sponge, etc. Smaller void ratio of the structured body leads to smaller amount of water absorbed and smaller flow rate of steam delivered. Thus the void ratio is preferably about 20 to 80%.
Micro-pores should preferably be distributed evenly. If the micro-pores are distributed unevenly, water absorbed in a concentrated area is last to be evaporated and steam is generated non-uniformly from the structured body, which deteriorates the steam generating efficiency.
It is undesirable for the supplied water to spill into the waveguide. It is also undesirable for the generated steam to travel through the waveguide and reach the microwave generating means. In the sixth steam generator, water is supplied to the container from the water inlet and is absorbed by the structured body. The steam generated in the structured body is discharged from the steam outlet of the container. When water is supplied to the structured body more than it can absorb, the water spill into the container, and is drained from the drain port. Thus, in the sixth steam generator, there is no fear of spilling out of water or steam in the waveguide.
If the container itself is heated by the microwave radiation, the heating efficiency of water lowers. In the seventh steam generator, the container is made of quartz which has a low dielectric constant and low dielectric dissipation factor (loss angle), so that it is hardly heated by the microwave radiation. Therefore, the energy for the water absorbed in the structured body is not wasted, and the water is effectively evaporated.